JP2016089799A - Abnormality diagnosis device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an abnormality detection device which can discriminate kinds of abnormalities occurring in air-fuel ratio sensors.SOLUTION: An abnormality diagnosis device of air-fuel ratio sensors 40, 41 which is arranged at an exhaust passage of an internal combustion engine, and generates a limit current corresponding to an air-fuel ratio comprises a current detection part 61 which detects output currents of the air-fuel ratio sensors, and an application voltage control device 60 which controls application voltages applied to the air-fuel ratio sensors. The abnormality diagnosis device applies a voltage within a limit current region in which the limit currents are generated at the air-fuel ratio sensors when exhaust air-fuel ratios around the air-fuel ratio sensors reach preset constant air-fuel ratios, and a voltage out of the limit current region, and determines kinds of abnormalities occurring in the air-fuel ratio sensors on the basis of output currents of the air-fuel ratio sensors which are detected by a current detection part at that time.SELECTED DRAWING: Figure 11

Description

  The present invention relates to an abnormality diagnosis device for an air-fuel ratio sensor disposed in an exhaust passage of an internal combustion engine.

  Conventionally, in an internal combustion engine in which the air-fuel ratio is controlled to a target air-fuel ratio, it is known that a limit current type air-fuel ratio sensor that generates a limit current corresponding to the air-fuel ratio is disposed in the engine exhaust passage. . In such an internal combustion engine, the amount of fuel supplied to the combustion chamber is feedback-controlled by the air-fuel ratio sensor so that the air-fuel ratio becomes the target air-fuel ratio. However, in this air-fuel ratio sensor, there is a case where element cracking occurs such that the outer surface of the sensor element communicates with the internal space of the sensor element. When such element cracking occurs, the air-fuel ratio sensor cannot generate an appropriate output corresponding to the air-fuel ratio, and as a result, the air-fuel ratio cannot be accurately feedback-controlled to the target air-fuel ratio.

  Therefore, an abnormality diagnosis device for detecting element cracks in an air-fuel ratio sensor has been conventionally known (for example, Patent Document 1). According to Patent Document 1, the applied voltage to the air-fuel ratio sensor is normally set at the center of the limit current region, and when the sensor element of the air-fuel ratio sensor is cracked or platinum on the electrode is condensed In other words, the voltage applied to the air-fuel ratio sensor is shifted from the center of the limit current region to the high voltage side. Therefore, in the apparatus described in Patent Document 1, when the applied voltage to the air-fuel ratio sensor is shifted from the center of the limit current region to the high voltage side or the low voltage side, the sensor element of the air-fuel ratio sensor is cracked. It is determined that it has occurred or platinum on the electrode has condensed.

JP 2010-174790 A Japanese Patent Laid-Open No. 10-062376 JP 2007-017191 A JP 2000-55861 A

  By the way, there are various abnormalities that occur in the air-fuel ratio sensor. Examples of such abnormalities include deterioration such as clogging in the diffusion rate limiting layer constituting the air-fuel ratio sensor, failure in a circuit connected to the air-fuel ratio sensor, and the like. Of these, when the diffusion rate-determining layer is clogged or deteriorated, an inclination shift occurs in which the degree of change in the output current of the air-fuel ratio sensor with respect to the change in the air-fuel ratio of the exhaust gas around the air-fuel ratio sensor deviates. On the other hand, when a failure occurs in a circuit connected to the air-fuel ratio sensor, an offset deviation occurs in which the output current of the air-fuel ratio sensor with respect to the air-fuel ratio of the exhaust gas around the air-fuel ratio sensor is entirely shifted by a constant value. However, with the conventional abnormality detection method, even if it is possible to detect that a deviation has occurred in the air-fuel ratio sensor, it has not been possible to determine whether it is an inclination deviation or an offset deviation. That is, the type of abnormality that has occurred in the air-fuel ratio sensor could not be determined.

  In view of the above problems, an object of the present invention is to provide an abnormality detection device that can determine the type of abnormality occurring in an air-fuel ratio sensor.

  In order to solve the above-mentioned problem, in the first invention, in an abnormality diagnosis device for an air-fuel ratio sensor that is provided in an exhaust passage of an internal combustion engine and generates a limit current corresponding to the air-fuel ratio, the output current of the air-fuel ratio sensor is A current detection unit for detecting, and an applied voltage control device for controlling an applied voltage to the air-fuel ratio sensor, wherein the air-fuel ratio of the exhaust gas flowing around the air-fuel ratio sensor is set to a predetermined constant air A voltage within a limit current region where a limit current is generated in the air-fuel ratio sensor when the fuel ratio is set and a voltage outside the limit current region are applied, and the output current of the air-fuel ratio sensor detected by the current detection unit at this time is applied. An abnormality diagnosis device for an air-fuel ratio sensor is provided that determines the type of abnormality occurring in the air-fuel ratio sensor based on the above.

  According to a second aspect, in the first aspect, the voltage outside the limit current region is a voltage within a proportional region that is lower than the limit current region and in which the output current increases as the applied voltage increases.

  In a third invention, in the first or second invention, when the air-fuel ratio sensor is normal, the air-fuel ratio of the exhaust gas flowing around the air-fuel ratio sensor is set to the predetermined constant air-fuel ratio. When the voltage within the limit current region is applied to the air-fuel ratio sensor in a maintained state and when the voltage outside the limit current region is applied, the output current is a normal value within the limit current region and a normal value outside the limit current region, respectively. In a state where the air-fuel ratio of the exhaust gas detected or calculated in advance and flowing around the air-fuel ratio sensor is maintained at the predetermined constant air-fuel ratio, the air-fuel ratio sensor is supplied with the voltage in the limit current region and the air-fuel ratio sensor. Based on the difference between the detected value of the output current of the air-fuel ratio sensor when a voltage outside the limit current region is applied and the normal value within the limit current region and the normal value outside the limit current region, To determine the type of abnormality are.

  According to a fourth aspect, in the third aspect, the air-fuel ratio sensor has the limit current region in a state where the air-fuel ratio of the exhaust gas flowing around the air-fuel ratio sensor is maintained at the predetermined constant air-fuel ratio. The difference between the detected value of the output current of the air-fuel ratio sensor when the voltage is applied and the normal value in the limit current region is greater than or equal to a predetermined reference value in the limit current region, and the air-fuel ratio sensor Detection of an output current of the air-fuel ratio sensor when a voltage outside the limit current region is applied to the air-fuel ratio sensor in a state where the air-fuel ratio of the exhaust gas flowing around is maintained at a predetermined constant air-fuel ratio If the difference between the value and the normal value outside the limit current region is equal to or greater than a predetermined reference value outside the limit current region, the air / fuel ratio is greater than the air / fuel ratio of the exhaust gas flowing around the air / fuel ratio sensor. Sensor output current is Determining an offset shift deviates the body specifically occurs in the air-fuel ratio sensor.

  According to a fifth aspect of the invention, in the third or fourth aspect of the invention, the air-fuel ratio sensor is configured so that the air-fuel ratio of the exhaust gas flowing around the air-fuel ratio sensor is maintained at the predetermined constant air-fuel ratio. The difference between the detected value of the output current of the air-fuel ratio sensor when a voltage in the limit current region is applied and the normal value in the limit current region is equal to or greater than a predetermined reference value in the limit current region, and When the air-fuel ratio of the exhaust gas flowing around the air-fuel ratio sensor is maintained at the predetermined constant air-fuel ratio, a voltage outside the limit current region is applied to the air-fuel ratio sensor. When the difference between the detected value of the output current and the normal value outside the limit current region is less than a predetermined reference value outside the limit current region, the change in the air-fuel ratio of the exhaust gas flowing around the air-fuel ratio sensor Said air-fuel ratio with respect to It is determined that the inclination shift has occurred in the air-fuel ratio sensor degree of change of the output current of capacitors is shifted.

  According to a sixth aspect, in the first to fifth aspects, the internal combustion engine is disposed in an exhaust purification catalyst disposed in an exhaust passage thereof and in the exhaust passage upstream of the exhaust purification catalyst in the exhaust flow direction. An upstream air-fuel ratio sensor, and a downstream air-fuel ratio sensor disposed in the exhaust passage downstream of the exhaust purification catalyst in the exhaust flow direction, the downstream air-fuel ratio sensor being the limit current type air-fuel ratio sensor Consists of.

  According to a seventh invention, in any one of the first to fifth inventions, the internal combustion engine includes an exhaust purification catalyst disposed in an exhaust passage thereof, and the exhaust passage upstream of the exhaust purification catalyst in the exhaust flow direction. An upstream air-fuel ratio sensor, and a downstream air-fuel ratio sensor disposed in the exhaust passage downstream of the exhaust purification catalyst in the exhaust flow direction. Air-fuel ratio sensor.

  In an eighth invention, in any one of the first to seventh inventions, the internal combustion engine can execute fuel cut control for stopping the supply of fuel to the combustion chamber during operation of the internal combustion engine, When the air-fuel ratio of the exhaust gas flowing around the air-fuel ratio sensor is maintained at the predetermined constant air-fuel ratio, the fuel cut control is being executed.

  According to a ninth invention, in the seventh invention, the internal combustion engine includes a fuel cut control for stopping the supply of fuel to the combustion chamber during operation of the internal combustion engine, and the exhaust purification catalyst after the fuel cut control ends. And a rich control after return for controlling the air-fuel ratio of the exhaust gas flowing into the exhaust gas to a rich air-fuel ratio richer than the stoichiometric air-fuel ratio, and the air-fuel ratio of the exhaust gas flowing around the air-fuel ratio sensor is determined in advance. When the constant air-fuel ratio is maintained, the post-return rich control is being executed.

  According to a tenth aspect, in the seventh aspect, the internal combustion engine performs feedback control so that the output air-fuel ratio of the upstream air-fuel ratio sensor becomes a target air-fuel ratio, and flows around the air-fuel ratio sensor. The time when the air-fuel ratio of the exhaust gas is maintained at the predetermined constant air-fuel ratio is when the target air-fuel ratio is maintained constant at the predetermined air-fuel ratio.

  In an eleventh aspect based on the seventh aspect, the internal combustion engine performs feedback control so that the output air-fuel ratio of the upstream air-fuel ratio sensor becomes a target air-fuel ratio, and flows around the air-fuel ratio sensor. When the air-fuel ratio of the exhaust gas is maintained at the predetermined constant air-fuel ratio, the oxygen storage amount of the exhaust purification catalyst is maintained at an amount greater than zero and less than the maximum storable oxygen amount. Thus, the target air-fuel ratio is alternately changed between a rich air-fuel ratio richer than the stoichiometric air-fuel ratio and a lean air-fuel ratio leaner than the stoichiometric air-fuel ratio.

  ADVANTAGE OF THE INVENTION According to this invention, the abnormality detection apparatus which can discriminate | determine the kind of abnormality which has arisen in the air fuel ratio sensor is provided.

FIG. 1 is a diagram schematically showing an internal combustion engine in which an abnormality diagnosis apparatus according to the present invention is used. FIG. 2 is a schematic cross-sectional view of the air-fuel ratio sensor. FIG. 3 is a diagram showing the relationship between the applied voltage V and the output current I at each exhaust air-fuel ratio A / F. FIG. 4 is a diagram showing the relationship between the air-fuel ratio and the output current I when the applied voltage V is constant. FIG. 5 is a time chart showing changes in the oxygen storage amount and the like of the upstream side exhaust purification catalyst during normal operation of the internal combustion engine. FIG. 6 shows the relationship between the exhaust air-fuel ratio and the output current of the air-fuel ratio sensor when the air-fuel ratio sensor is normal and when an abnormality occurs. FIG. 7 is a diagram showing the relationship between the voltage applied to the air-fuel ratio sensor and the output current. FIG. 8 is a diagram showing the relationship between the voltage applied to the air-fuel ratio sensor and the output current. FIG. 9 is a diagram showing the relationship between the voltage applied to the air-fuel ratio sensor and the output current. FIG. 10 is a schematic cross-sectional view of an air-fuel ratio sensor in which element cracking has occurred. FIG. 11 is a time chart showing changes in the output air-fuel ratio of the downstream air-fuel ratio sensor when abnormality diagnosis is performed. FIG. 12 is a flowchart for performing abnormality diagnosis of the downstream air-fuel ratio sensor. FIG. 13 is a flowchart showing a change in the output air-fuel ratio of the downstream air-fuel ratio sensor when abnormality diagnosis is performed. FIG. 14 is a flowchart for performing abnormality diagnosis of the downstream air-fuel ratio sensor.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the following description, the same reference numerals are assigned to similar components.

<Description of the internal combustion engine as a whole>
FIG. 1 is a diagram schematically showing an internal combustion engine in which an abnormality diagnosis apparatus according to a first embodiment of the present invention is used. Referring to FIG. 1, 1 is an engine body, 2 is a cylinder block, 3 is a piston that reciprocates in the cylinder block 2, 4 is a cylinder head fixed on the cylinder block 2, and 5 is a piston 3 and a cylinder head 4. A combustion chamber formed therebetween, 6 is an intake valve, 7 is an intake port, 8 is an exhaust valve, and 9 is an exhaust port. The intake valve 6 opens and closes the intake port 7, and the exhaust valve 8 opens and closes the exhaust port 9.

  As shown in FIG. 1, a spark plug 10 is disposed at the center of the inner wall surface of the cylinder head 4, and a fuel injection valve 11 is disposed around the inner wall surface of the cylinder head 4. The spark plug 10 is configured to generate a spark in response to the ignition signal. The fuel injection valve 11 injects a predetermined amount of fuel into the combustion chamber 5 according to the injection signal. The fuel injection valve 11 may be arranged so as to inject fuel into the intake port 7. In this embodiment, gasoline having a theoretical air-fuel ratio of 14.6 is used as the fuel. However, in the internal combustion engine in which the abnormality diagnosis device of the present invention is used, a fuel other than gasoline or a mixed fuel with gasoline may be used.

  The intake port 7 of each cylinder is connected to a surge tank 14 via a corresponding intake branch pipe 13, and the surge tank 14 is connected to an air cleaner 16 via an intake pipe 15. The intake port 7, the intake branch pipe 13, the surge tank 14, and the intake pipe 15 form an intake passage. A throttle valve 18 driven by a throttle valve drive actuator 17 is disposed in the intake pipe 15. The throttle valve 18 is rotated by a throttle valve drive actuator 17 so that the opening area of the intake passage can be changed.

  On the other hand, the exhaust port 9 of each cylinder is connected to an exhaust manifold 19. The exhaust manifold 19 has a plurality of branches connected to the exhaust ports 9 and a collective part in which these branches are assembled. A collecting portion of the exhaust manifold 19 is connected to an upstream casing 21 containing an upstream exhaust purification catalyst 20. The upstream casing 21 is connected to a downstream casing 23 containing a downstream exhaust purification catalyst 24 via an exhaust pipe 22. The exhaust port 9, the exhaust manifold 19, the upstream casing 21, the exhaust pipe 22, and the downstream casing 23 form an exhaust passage.

  An electronic control unit (ECU) 31 comprises a digital computer, and is connected to each other via a bidirectional bus 32, a RAM (Random Access Memory) 33, a ROM (Read Only Memory) 34, a CPU (Microprocessor) 35, and an input. A port 36 and an output port 37 are provided. An air flow meter 39 for detecting the flow rate of air flowing through the intake pipe 15 is disposed in the intake pipe 15, and the output of the air flow meter 39 is input to the input port 36 via the corresponding AD converter 38. Further, an upstream air-fuel ratio sensor 40 that detects the air-fuel ratio of the exhaust gas flowing through the exhaust manifold 19 (that is, the exhaust gas flowing into the upstream exhaust purification catalyst 20) is disposed at the collecting portion of the exhaust manifold 19. In addition, in the exhaust pipe 22, the downstream side that detects the air-fuel ratio of the exhaust gas that flows in the exhaust pipe 22 (that is, the exhaust gas that flows out of the upstream side exhaust purification catalyst 20 and flows into the downstream side exhaust purification catalyst 24). An air-fuel ratio sensor 41 is arranged. The outputs of these air-fuel ratio sensors 40 and 41 are also input to the input port 36 via the corresponding AD converter 38. The configuration of these air-fuel ratio sensors 40 and 41 will be described later.

  A load sensor 43 that generates an output voltage proportional to the amount of depression of the accelerator pedal 42 is connected to the accelerator pedal 42, and the output voltage of the load sensor 43 is input to the input port 36 via the corresponding AD converter 38. The For example, the crank angle sensor 44 generates an output pulse every time the crankshaft rotates 15 degrees, and this output pulse is input to the input port 36. The CPU 35 calculates the engine speed from the output pulse of the crank angle sensor 44. On the other hand, the output port 37 is connected to the spark plug 10, the fuel injection valve 11, and the throttle valve drive actuator 17 via the corresponding drive circuit 45. The ECU 31 functions as an abnormality diagnosis device that performs abnormality diagnosis of the downstream air-fuel ratio sensor 41.

The upstream side exhaust purification catalyst 20 and the downstream side exhaust purification catalyst 24 are three-way catalysts having oxygen storage capacity. Specifically, the exhaust purification catalysts 20 and 24 support a noble metal having a catalytic action (for example, platinum (Pt)) and a substance having an oxygen storage capacity (for example, ceria (CeO ₂ )) on a ceramic support. Three-way catalyst. The three-way catalyst has a function of simultaneously purifying unburned HC, CO, and NOx when the air-fuel ratio of the exhaust gas flowing into the three-way catalyst is maintained at the stoichiometric air-fuel ratio. In addition, when the exhaust purification catalysts 20 and 24 have an oxygen storage capacity, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalysts 20 and 24 is slightly richer or leaner than the stoichiometric air-fuel ratio. Even if it deviates, unburned HC, CO and NOx are simultaneously purified.

  That is, if the exhaust purification catalysts 20, 24 have oxygen storage capacity, when the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalysts 20, 24 becomes slightly leaner than the stoichiometric air-fuel ratio, Excess oxygen contained is occluded in the exhaust purification catalysts 20, 24, and the surfaces of the exhaust purification catalysts 20, 24 are maintained at the stoichiometric air-fuel ratio. As a result, unburned HC, CO, and NOx are simultaneously purified on the surfaces of the exhaust purification catalysts 20, 24, and at this time, the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalysts 20, 24 becomes the stoichiometric air-fuel ratio.

  On the other hand, when the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalysts 20, 24 becomes slightly richer than the stoichiometric air-fuel ratio, it is insufficient to reduce unburned HC and CO contained in the exhaust gas. The released oxygen is released from the exhaust purification catalysts 20, 24, and in this case as well, the surfaces of the exhaust purification catalysts 20, 24 are maintained at the stoichiometric air-fuel ratio. As a result, unburned HC, CO, and NOx are simultaneously purified on the surfaces of the exhaust purification catalysts 20, 24, and at this time, the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalysts 20, 24 becomes the stoichiometric air-fuel ratio.

  As described above, when the exhaust purification catalysts 20 and 24 have the oxygen storage capacity, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalysts 20 and 24 is richer or leaner than the stoichiometric air-fuel ratio. Even if there is a slight deviation, unburned HC, CO and NOx are simultaneously purified, and the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalysts 20, 24 becomes the stoichiometric air-fuel ratio.

<Description of air-fuel ratio sensor>
In the present embodiment, a cup-type limit current type air-fuel ratio sensor is used as the air-fuel ratio sensors 40 and 41. The structure of the air-fuel ratio sensors 40 and 41 will be briefly described with reference to FIG. The air-fuel ratio sensors 40 and 41 include a solid electrolyte layer 51, an exhaust-side electrode 52 disposed on one side surface thereof, an atmosphere-side electrode 53 disposed on the other side surface, and diffusion of exhaust gas passing therethrough. A diffusion rate controlling layer 54 for controlling the rate, a reference gas chamber 55, and a heater unit 56 for heating the air-fuel ratio sensors 40 and 41, particularly for heating the solid electrolyte layer 51 are provided.

  In particular, in the cup-type air-fuel ratio sensors 40 and 41 of the present embodiment, the solid electrolyte layer 51 is formed in a cylindrical shape with one end closed. In the reference gas chamber 55 defined inside the solid electrolyte layer 51, atmospheric gas (air) is introduced and a heater unit 56 is disposed. An atmosphere side electrode 53 is disposed on the inner surface of the solid electrolyte layer 51, and an exhaust side electrode 52 is disposed on the outer surface of the solid electrolyte layer 51. On the outer surfaces of the solid electrolyte layer 51 and the exhaust-side electrode 52, a diffusion control layer 54 is disposed so as to cover them. A protective layer (not shown) for preventing liquid or the like from adhering to the surface of the diffusion limiting layer 54 may be provided outside the diffusion limiting layer 54.

The solid electrolyte layer 51 is an oxygen ion conductive oxide in which ZrO ₂ (zirconia), HfO ₂ , ThO ₂ , Bi ₂ O _3, etc. are distributed with CaO, MgO, Y ₂ O ₃ , Yb ₂ O _3, etc. as stabilizers. The sintered body is formed. The diffusion control layer 54 is formed of a porous sintered body of a heat-resistant inorganic substance such as alumina, magnesia, silica, spinel, mullite or the like. Furthermore, the exhaust-side electrode 52 and the atmosphere-side electrode 53 are formed of a noble metal having high catalytic activity such as platinum.

  Further, a sensor applied voltage V is applied between the exhaust side electrode 52 and the atmosphere side electrode 53 by the applied voltage control device 60 mounted on the ECU 31. In addition, the ECU 31 is provided with a current detection unit 61 that detects a current I flowing between the electrodes 52 and 53 via the solid electrolyte layer 51 when the sensor application voltage V is applied. The current detected by the current detector 61 is the output current I of the air-fuel ratio sensors 40 and 41.

The thus configured air-fuel ratio sensors 40 and 41 have voltage-current (V-I) characteristics as shown in FIG. As can be seen from FIG. 3, the output current I of the air-fuel ratio sensors 40 and 41 increases as the air-fuel ratio of the exhaust gas, that is, the exhaust air-fuel ratio A / F increases (lean). The V-I line at each exhaust air-fuel ratio A / F includes a region parallel to the sensor applied voltage V axis, that is, a region where the output current I hardly changes even when the sensor applied voltage V changes. This voltage region is referred to as a limiting current region, and the current at this time is referred to as a limiting current. In FIG. 3, the limit current region and limit current when the exhaust air-fuel ratio is 18 are indicated by W ₁₈ and I ₁₈ , respectively.

  On the other hand, in a region where the sensor applied voltage is lower than the limit current region, the output current rises in proportion to the increase in the sensor applied voltage. Such a region is called a proportional region. The inclination at this time is determined by the DC element resistance of the solid electrolyte layer 51. Further, in a region where the sensor applied voltage is higher than the limit current region, the output current increases as the sensor applied voltage increases. In this region, the output voltage changes according to the change in the sensor applied voltage due to, for example, decomposition of moisture contained in the exhaust gas on the exhaust side electrode 52.

  FIG. 4 shows the relationship between the exhaust air-fuel ratio and the output current I when the applied voltage V is kept constant at about 0.45 V (FIG. 3). As can be seen from FIG. 4, in the air-fuel ratio sensors 40 and 41, the exhaust air-fuel ratio becomes higher with respect to the exhaust air-fuel ratio so that the output current I from the air-fuel ratio sensors 40 and 41 becomes larger as the exhaust air-fuel ratio becomes higher (that is, the leaner). The output current changes linearly (in proportion). In addition, the air-fuel ratio sensors 40 and 41 are configured such that the output current I becomes zero when the exhaust air-fuel ratio is the stoichiometric air-fuel ratio.

  As the air-fuel ratio sensors 40 and 41, instead of the limit current type air-fuel ratio sensor having the structure shown in FIG. 2, for example, a limit current type air-fuel ratio having another structure such as a stacked type limit current type air-fuel ratio sensor is used. A sensor may be used.

<Basic control>
In the internal combustion engine configured as described above, the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is based on the engine operating state based on the outputs of the upstream side air-fuel ratio sensor 40 and the downstream side air-fuel ratio sensor 41. The fuel injection amount from the fuel injection valve 11 is set so as to achieve an optimal air-fuel ratio. As such a method for setting the fuel injection amount, the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 based on the output of the upstream side air-fuel ratio sensor 40 (or the target air flow of the exhaust gas flowing out from the engine body). (Fuel ratio) is controlled so as to become the target air-fuel ratio, and the output of the upstream air-fuel ratio sensor 40 is corrected based on the output of the downstream air-fuel ratio sensor 41, or the target air-fuel ratio is changed. It is done.

  With reference to FIG. 5, an example of such control of the target air-fuel ratio will be briefly described. FIG. 5 is a time chart of the oxygen storage amount of the upstream side exhaust purification catalyst, the target air-fuel ratio, the output air-fuel ratio of the upstream air-fuel ratio sensor, and the output air-fuel ratio of the downstream air-fuel ratio sensor during normal operation of the internal combustion engine. . “Output air-fuel ratio” means an air-fuel ratio corresponding to the output of the air-fuel ratio sensor. The “normal operation” is a control for adjusting the fuel injection amount in accordance with a specific operation state of the internal combustion engine (for example, an increase correction of the fuel injection amount performed when the vehicle equipped with the internal combustion engine is accelerated, a combustion chamber) This means an operation state (control state) in which fuel cut control for stopping the supply of fuel to the vehicle is not performed.

  In the example shown in FIG. 5, when the output air-fuel ratio of the downstream air-fuel ratio sensor 41 becomes equal to or less than the rich determination air-fuel ratio AFrich (for example, 14.55), the target air-fuel ratio is set to the lean set air-fuel ratio AFTlean (for example, for example). 15) and maintained. Thereafter, the oxygen storage amount of the upstream side exhaust purification catalyst 20 is estimated, and when this estimated value is equal to or greater than a predetermined reference storage amount Cref (an amount smaller than the maximum oxygen storage amount Cmax), the target air-fuel ratio is set to a rich setting. The air-fuel ratio AFTrich (for example, 14.4) is set and maintained. In the example shown in FIG. 5, such an operation is repeatedly performed.

Specifically, in the example shown in FIG. 5, before the time t ₁ , the target air-fuel ratio is set to the rich set air-fuel ratio AFTrich, and accordingly, the output air-fuel ratio of the upstream air-fuel ratio sensor 40 is also the theoretical air-fuel ratio. The air-fuel ratio is richer than the fuel ratio (hereinafter referred to as “rich air-fuel ratio”). Further, since oxygen is stored in the upstream side exhaust purification catalyst 20, the output air-fuel ratio of the downstream side air-fuel ratio sensor 41 is substantially the stoichiometric air-fuel ratio (14.6). At this time, since the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is a rich air-fuel ratio, the oxygen storage amount of the upstream side exhaust purification catalyst 20 gradually decreases.

Thereafter, at time t ₁ , when the oxygen storage amount of the upstream side exhaust purification catalyst 20 approaches zero, a part of the unburned gas (unburned HC, CO) flowing into the upstream side exhaust purification catalyst 20 is upstream. It begins to flow out without being purified by the exhaust purification catalyst 20. As a result, at the time t ₂ , the output air-fuel ratio of the downstream side air-fuel ratio sensor 41 becomes the rich determination air-fuel ratio AFrich that is slightly richer than the stoichiometric air-fuel ratio, and at this time, the target air-fuel ratio becomes lean from the rich set air-fuel ratio AFTrich to the lean set air-fuel ratio. It is switched to the fuel ratio AFTlean.

By switching the target air-fuel ratio, the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 becomes an air-fuel ratio leaner than the stoichiometric air-fuel ratio (hereinafter referred to as “lean air-fuel ratio”). Decrease and stop. The oxygen storage amount of the upstream exhaust purifying catalyst 20b is gradually increased, at time t _3, reaches the determination reference storage amount Cref. Thus, when the oxygen storage amount reaches the determination reference storage amount Cref, the target air-fuel ratio is again switched from the lean set air-fuel ratio AFlena to the rich set air-fuel ratio AFTrich. By switching the target air-fuel ratio, the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 becomes a rich air-fuel ratio again. As a result, the oxygen storage amount of the upstream side exhaust purification catalyst 20 gradually decreases, and thereafter Such an operation is repeated. By performing such control, it is possible to prevent NOx from flowing out of the upstream side exhaust purification catalyst 20.

  The control of the target air-fuel ratio based on the outputs of the upstream air-fuel ratio sensor 40 and the downstream air-fuel ratio sensor 41 that is performed as normal control is not limited to the control described above. Any control may be used as long as the control is based on the outputs of these air-fuel ratio sensors 40 and 41. Therefore, for example, as the normal control, the target air-fuel ratio is fixed to the stoichiometric air-fuel ratio, feedback control is performed so that the output air-fuel ratio of the upstream air-fuel ratio sensor 40 becomes the stoichiometric air-fuel ratio, and the downstream air-fuel ratio sensor 41 Control may be performed to correct the output air-fuel ratio of the upstream air-fuel ratio sensor 40 based on the output air-fuel ratio.

<Problems in air-fuel ratio sensor abnormality diagnosis>
By the way, various output abnormalities may occur in the air-fuel ratio sensors 40 and 41. As such an output abnormality, for example, the one shown in FIG. 6 can be considered. FIG. 6 shows the relationship between the exhaust air-fuel ratio and the output current of the air-fuel ratio sensors 40, 41 when the air-fuel ratio sensors 40, 41 are normal and abnormal. The broken line in FIG. 6 shows the relationship when there is no abnormality in the air-fuel ratio sensors 40, 41, while the solid line in FIG. 6 shows the relationship when there is an abnormality in the air-fuel ratio sensors 40, 41. Yes.

  In the case indicated by X in FIG. 6, a deviation that causes the output currents of the air-fuel ratio sensors 40 and 41 to be smaller (or larger) than an appropriate value in the entire exhaust air-fuel ratio, that is, an offset deviation occurs. Shows the case. Therefore, in this case, the output current I of the air-fuel ratio sensors 40 and 41 indicates the air-fuel ratio richer (or leaner) than the actual air-fuel ratio in the entire region. On the other hand, in the case shown by Y in FIG. 6, a deviation in which the degree of change of the output current I of the air-fuel ratio sensors 40 and 41 with respect to the change of the exhaust air-fuel ratio becomes larger (or smaller) than an appropriate value, The case where the inclination shift has occurred is shown. That is, the slope of the output current I with respect to the exhaust air / fuel ratio in the example indicated by Y in FIG. 6 is a large value with respect to the slope of the normal air / fuel ratio sensors 40 and 41. Therefore, in this case, the absolute value of the output current of the air-fuel ratio sensors 40 and 41 indicates a rich degree or lean degree that is larger (or smaller) than the rich degree or lean degree in the actual air-fuel ratio.

  Here, when performing the normal control as shown in FIG. 5, the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 by the upstream side air-fuel ratio sensor 40 is a rich air-fuel ratio or a lean air-fuel ratio. It is important to accurately detect whether or not. As shown in FIG. 5, when the target air-fuel ratio is a rich air-fuel ratio, the actual air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is a lean air-fuel ratio. This is because normal control is no longer established. Similarly, it is important to detect whether the air-fuel ratio of the exhaust gas flowing out from the upstream side exhaust purification catalyst 20 is in the vicinity of the stoichiometric air-fuel ratio, the rich air-fuel ratio, or the lean air-fuel ratio by the downstream air-fuel ratio sensor 41. is there. This is because the air-fuel ratio detected by the downstream air-fuel ratio sensor 41 is a rich air-fuel ratio even though the actual air-fuel ratio of the exhaust gas flowing out from the upstream side exhaust purification catalyst 20 is the stoichiometric air-fuel ratio. This is because the normal control as shown in FIG.

  Therefore, when performing normal control, the exhaust air-fuel ratio is higher than the stoichiometric air-fuel ratio than the richness or leanness of the exhaust air-fuel ratio on the upstream side and downstream side of the upstream side exhaust purification catalyst 20. It is necessary to accurately detect whether it is rich or lean. For this reason, when the offset deviation indicated by X in FIG. 6 occurs, a deviation occurs in the output current at the stoichiometric air-fuel ratio. Therefore, it is necessary to detect an abnormality even if the deviation is slight. However, even if the deviation is slight, when an offset deviation is detected, the offset deviation occurs not only when the offset deviation occurs but also when the inclination deviation as indicated by Y in FIG. 6 occurs. It may be judged that Therefore, if the abnormality diagnosis of the air-fuel ratio sensors 40 and 41 is performed based only on the relationship between the exhaust air-fuel ratio and the output current I, the type of abnormality (abnormal mode) that has occurred may not be accurately identified.

<Characteristics of abnormality in air-fuel ratio sensor>
Incidentally, the relationship between the voltage V applied to the air-fuel ratio sensors 40 and 41 and the output current I varies depending on the type of abnormality occurring in the air-fuel ratio sensors 40 and 41. FIG. 7 shows the application to the air-fuel ratio sensors 40 and 41 in a state where the atmospheric gas is circulating around the air-fuel ratio sensors 40 and 41 (that is, the exhaust gas having an air-fuel ratio corresponding to the atmospheric gas is circulating). The relationship between the voltage V and the output current I is shown. The solid line in FIG. 7 shows the relationship in the case where an abnormality has occurred in circuits such as the applied voltage control device 60 and the current detection unit 61 of the air-fuel ratio sensors 40 and 41. On the other hand, the broken line in FIG. 7 shows the relationship when there is no abnormality in the air-fuel ratio sensors 40 and 41, that is, when it is normal.

As shown in FIG. 7, when an abnormality occurs in the circuits of the air-fuel ratio sensors 40 and 41, the output current I increases by a constant value over the entire range of the applied voltage V compared to the normal case. doing. As a result, when the applied voltage V ₂ in the limit current region Wlc is applied to the air-fuel ratio sensors 40 and 41, the output current I when the air-fuel ratio sensors 40 and 41 are abnormal is normal. It is higher than I by a certain value. Similarly, when the applied voltage V ₁ within the proportional region Wip is applied to the air-fuel ratio sensors 40, 41, the output current I when the air-fuel ratio sensors 40, 41 are normal is the output current when the air-fuel ratio sensors 40, 41 are normal. It is higher than I by a certain value. The limit current region Wlc indicates a limit current region that is generated when air gas is circulating around the air-fuel ratio sensors 40, 41 when no abnormality occurs in the air-fuel ratio sensors 40, 41. . Similarly, the proportional region Wip indicates a proportional region that occurs when air gas is circulating around the air-fuel ratio sensors 40, 41 when no abnormality occurs in the air-fuel ratio sensors 40, 41.

  Therefore, when an abnormality occurs in the circuits of the air-fuel ratio sensors 40, 41, etc., even when the voltage I applied to the air-fuel ratio sensors 40, 41 is a voltage in the limit current region Wlc, the voltage in the proportional region Wip Even when the output current I is, the output current I increases as compared with the normal case. In the illustrated example, the output current I is increased due to an abnormality in the circuits of the air-fuel ratio sensors 40 and 41, but the output current I is spread over the entire area due to an abnormality in the circuits of the air-fuel ratio sensors 40 and 41. It may decrease over time.

  As described above, when an abnormality occurs in the circuits of the air-fuel ratio sensors 40 and 41, the output current I of the air-fuel ratio sensors 40 and 41 is always a value deviated from the original value by a constant value. As a result, when an abnormality occurs in the circuits of the air-fuel ratio sensors 40 and 41, the relationship between the exhaust air-fuel ratio around the air-fuel ratio sensors 40 and 41 and the output current I is as indicated by X in FIG. Thus, an offset deviation in which the output current I is shifted to a value smaller than an appropriate value occurs in the entire exhaust air-fuel ratio.

  FIG. 8 also shows the relationship between the applied voltage V to the air-fuel ratio sensor 40, 41 and the output current I in the state where the atmospheric gas is circulating around the air-fuel ratio sensor 40, 41. A solid line in the figure indicates an abnormality such as partial clogging or cracking in the diffusion rate controlling layer 54 of the air-fuel ratio sensor 40 or 41, or an abnormality such as deterioration in the electrodes 52 or 53 of the air-fuel ratio sensor 40 or 41. The relationship in the case where this occurs is shown. On the other hand, the broken line in the figure indicates the relationship when no abnormality has occurred in the air-fuel ratio sensors 40 and 41.

As shown in FIG. 8, when an abnormality occurs in the diffusion rate limiting layer 54, the electrodes 52, 53, etc. of the air-fuel ratio sensors 40, 41, the output current is only in the limit current region Wlc as compared with the normal case. I increases by a certain value. As a result, when the applied voltage V ₂ in the limit current region Wlc is applied to the air-fuel ratio sensors 40 and 41, the output current I when the air-fuel ratio sensors 40 and 41 are abnormal is normal. It is higher than I by a certain value. On the other hand, when the applied voltage V ₁ within the proportional region Wip is applied to the air-fuel ratio sensors 40 and 41, the output current I when the air-fuel ratio sensors 40 and 41 are normal is also the output current when the air-fuel ratio sensors 40 and 41 are normal. I also has almost the same value. In the illustrated example, the output current I is increased due to an abnormality in the diffusion rate limiting layer 54 of the air / fuel ratio sensors 40 and 41, the electrodes 52 and 53, etc., but the diffusion rate limiting layer of the air / fuel ratio sensors 40 and 41 is shown. In some cases, the output current I may decrease due to an abnormality in the 54, the electrodes 52, 53, or the like.

  The reason why such a phenomenon occurs will be described by taking as an example the case where the diffusion rate controlling layer 54 is clogged or cracked. Here, the limit current as described above is caused by the diffusion-controlled layer 54. That is, the amount of oxygen ions that can move within the unit time in the solid electrolyte layer 51 is determined according to the applied voltage V, but in the proportional region, the amount of oxygen ions that can move within the unit time. However, the flow rate of the unburned gas and oxygen reaching the electrode 52 through the diffusion control layer 54 is larger (see FIG. 2). As a result, in the proportional region, as the applied voltage V increases, the amount of oxygen ions that move in the solid electrolyte layer 51 increases, and the output current I increases. For this reason, the inclination in the VI diagram at this time is determined according to the DC element resistance of the solid electrolyte layer 51.

  However, in the limiting current region, the flow rate of unburned gas and oxygen reaching the electrode 52 via the diffusion-controlling layer 54 is smaller than the amount of oxygen ions that can move within the solid electrolyte layer 51 within a unit time. . As a result, in the limiting current region, even if the applied voltage V changes, the amount of oxygen ions that move in the solid electrolyte layer 51 is the amount of unburned gas or oxygen that reaches the electrode 52 via the diffusion rate controlling layer 54. The flow rate remains constant. As a result, even if the applied voltage V changes in the limit current region, the amount of oxygen ions that move in the solid electrolyte layer 51 does not change, and therefore the output current I does not change.

  When such a diffusion-controlling layer 54 is clogged or cracked, the flow rate of unburned gas or oxygen reaching the electrode through the diffusion-controlling layer 54 changes. As a result, in the limit current region, the output current I changes because the output current I is determined by the flow rate of the unburned gas and oxygen reaching the electrode 52 through the diffusion rate controlling layer 54. On the other hand, as described above, in the proportional region, the amount of oxygen ions that can move within the solid electrolyte layer 51 within a unit time rather than the flow rate of unburned gas and oxygen reaching the electrode 52 through the diffusion rate controlling layer 54. There are more. As a result, the output current I in the proportional region does not change even if the diffusion rate controlling layer 54 is clogged or cracked.

  When the diffusion-controlling layer 54 is clogged or cracked, the degree to which the output current I changes is larger as the deviation of the exhaust air / fuel ratio from the stoichiometric air / fuel ratio becomes larger than when there is no such phenomenon. This is because, as the deviation of the exhaust air / fuel ratio from the stoichiometric air / fuel ratio increases, the amount of oxygen and unburned gas contained in the unit exhaust gas increases, and therefore the amount of exhaust gas passing through the diffusion rate controlling layer 54 changes. This is because the amount of unburned gas and oxygen reaching the electrode 52 varies greatly. As a result, when an abnormality occurs in the diffusion rate limiting layer 54 of the air-fuel ratio sensors 40 and 41, the electrodes 52 and 53, etc., a tilt shift as indicated by Y in FIG. 6 occurs.

  FIG. 9 also shows the relationship between the voltage V applied to the air-fuel ratio sensors 40 and 41 and the output current I in the state where the atmospheric gas is circulating around the air-fuel ratio sensors 40 and 41. The solid line in the figure shows the relationship when abnormality such as element cracking occurs in the air-fuel ratio sensors 40 and 41. Here, the element cracks of the air-fuel ratio sensors 40 and 41 are specifically the cracks penetrating the solid electrolyte layer 51 and the diffusion rate limiting layer 54 (C1 in FIG. 10), the solid electrolyte layer 51 and the diffusion rate limiting layer 54, or the like. In addition, it means a crack (C2 in FIG. 10) penetrating both electrodes 52 and 53. On the other hand, the broken line in the figure indicates the relationship when no abnormality has occurred in the air-fuel ratio sensors 40 and 41. Thus, if element cracks occur in the air-fuel ratio sensors 40 and 41, an abnormality (reference gas abnormality) occurs in the reference gas (usually atmospheric gas) in the reference gas chamber 55.

As shown in FIG. 9, when an abnormality occurs in the reference gas of the air-fuel ratio sensors 40 and 41, the output current I increases by a certain value only in the proportional region Wip, compared to the normal case. . As a result, when the applied voltage V ₂ in the limit current region Wlc is applied to the air-fuel ratio sensors 40, 41, the output current I when the air-fuel ratio sensors 40, 41 are abnormal is also normal. The output current I is also almost the same value. On the other hand, when the applied voltage V ₁ within the proportional region Wip is applied to the air-fuel ratio sensors 40 and 41, the output current I when the air-fuel ratio sensors 40 and 41 are abnormal is greater than the output current I when normal. Is also rising by a certain value.

From the above, the phenomena shown in FIGS. 7 to 9 can be summarized as shown in Table 1 below.

<Abnormality diagnosis control>
Therefore, in the present embodiment, current detection for detecting the output current I of the air-fuel ratio sensors 40 and 41 in the abnormality diagnosis device for the air-fuel ratio sensor that is provided in the exhaust passage of the internal combustion engine and generates a limit current corresponding to the air-fuel ratio. Unit 61 and an applied voltage control device 60 for controlling the applied voltage to the air-fuel ratio sensors 40 and 41, and the air-fuel ratio of the exhaust gas flowing around the air-fuel ratio sensors 40 and 41 is set to a predetermined level. A voltage within the limit current region where a limit current is generated and a voltage outside this limit current region (particularly the proportional region) are applied to the air-fuel ratio sensors 40 and 41 when the fuel ratio is set, and at this time, the current detection unit detects the voltage. The type of abnormality occurring in the air-fuel ratio sensors 40, 41 is determined based on the output current I of the air-fuel ratio sensors 40, 41. The application of the voltage within the limit current region and the application of the voltage outside the limit current region are performed, for example, in a state where the air-fuel ratio of the exhaust gas flowing around the air-fuel ratio sensors 40 and 41 is maintained at a constant air-fuel ratio. 60 is performed by changing the voltage applied to the air-fuel ratio sensors 40 and 41.

  In particular, in the present embodiment, when the air-fuel ratio sensors 40, 41 are normal, the air-fuel ratio of the exhaust gas flowing around the air-fuel ratio sensors 40, 41 is maintained at a predetermined constant air-fuel ratio. When the voltage within the limit current region is applied to the air-fuel ratio sensors 40 and 41 and when the voltage outside the limit current region is applied, the output currents are detected in advance as normal values within the limit current region and normal values outside the limit current region, respectively. The voltage within the limit current region is applied to the air-fuel ratio sensors 40, 41 in a state where the air-fuel ratio of the exhaust gas calculated and circulated around the air-fuel ratio sensors 40, 41 is maintained at a predetermined constant air-fuel ratio. The difference between the detected value of the output current of the air-fuel ratio sensor 40, 41 and the normal value in the limit current region, and the air-fuel ratio sensor 40, 41 when a voltage outside the limit current region is applied to the air-fuel ratio sensor 40, 41. And so as to determine the type of abnormality occurring in the air-fuel ratio sensor 40 and 41 based on the difference between the detected value and the limiting current region outside the normal value of the output current.

<Description of control using time chart>
Next, referring to the time chart shown in FIG. 11, the abnormality diagnosis of the air-fuel ratio sensor in the present embodiment will be described taking as an example the case where abnormality diagnosis of the downstream air-fuel ratio sensor 41 is performed. In the present embodiment, as already described with reference to FIG. 5, the target air-fuel ratio is normally changed alternately between the rich set air-fuel ratio AFTrich and the lean set air-fuel ratio AFTlean. Control in which the target air-fuel ratio is changed alternately between the rich set air-fuel ratio AFTrich and the lean set air-fuel ratio AFTlean is referred to as normal control.

  On the other hand, in the present embodiment, even when the crankshaft and the piston 3 are moving (ie, during operation of the internal combustion engine) during deceleration of the vehicle on which the internal combustion engine is mounted, the fuel injection valve 11 Fuel cut control for stopping the supply of fuel to the combustion chamber 5 is performed. Further, when the fuel cut control is performed, the oxygen storage amount of the exhaust purification catalysts 20, 24 reaches the maximum storable oxygen amount. For this reason, after the fuel cut control is finished, the rich control after returning to make the target air-fuel ratio richer than the rich set air-fuel ratio AFTrich during the normal control described above in order to release the oxygen stored in the exhaust purification catalysts 20, 24. Is done.

  Here, the abnormality diagnosis of the downstream air-fuel ratio sensor 41 in the present embodiment is performed when the air-fuel ratio of the exhaust gas around the downstream air-fuel ratio sensor 41 is maintained at a constant air-fuel ratio. In particular, in the present embodiment, abnormality diagnosis is performed during fuel cut control in which the air-fuel ratio of the exhaust gas around the downstream air-fuel ratio sensor 41 is maintained at the air-fuel ratio corresponding to the atmospheric gas. In addition, in the present embodiment, abnormality diagnosis is performed even during execution of post-return rich control in which the air-fuel ratio of the exhaust gas around the downstream air-fuel ratio sensor 41 is substantially the stoichiometric air-fuel ratio.

  FIG. 11 shows the presence or absence of these fuel cut control and post-return rich control, the target air-fuel ratio, the output air-fuel ratio of the upstream air-fuel ratio sensor 40, the output air-fuel ratio of the downstream air-fuel ratio sensor 41, and the downstream air-fuel ratio. 4 is a time chart of a voltage applied to a sensor 41 and a completion flag indicating completion of abnormality diagnosis.

In the example shown in FIG. 11, execution of fuel cut control is started at time t ₁ . Before execution of the fuel cut control at time t ₁ , the target air-fuel ratio becomes the rich set air-fuel ratio AFTrich during normal control in which the target air-fuel ratio is alternately changed to the rich air-fuel ratio and the lean air-fuel ratio. Shows the case. At this time, the output air-fuel ratio of the upstream air-fuel ratio sensor 40 is a rich air-fuel ratio. At this time, since the unburned gas in the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is purified by the upstream side exhaust purification catalyst 20, the output air-fuel ratio of the downstream side air-fuel ratio sensor 41 becomes the stoichiometric air-fuel ratio. ing.

When execution of the fuel cut control is started at time t ₁ , the atmospheric gas flows out from the engine body 1, so that the output air-fuel ratio AFup of the upstream air-fuel ratio sensor 40 has an extremely lean degree corresponding to the atmospheric gas. Change to lean air-fuel ratio. Further, although atmospheric gas also flows into the upstream side exhaust purification catalyst 20, oxygen in the atmospheric gas that has flowed into the upstream side exhaust purification catalyst 20 is occluded by the upstream side exhaust purification catalyst 20. For this reason, immediately after the start of the fuel cut control, the output air-fuel ratio of the downstream air-fuel ratio sensor 41 is maintained substantially at the stoichiometric air-fuel ratio. However, the oxygen storage amount of the upstream side exhaust purification catalyst 20 immediately reaches the maximum storable oxygen amount, and atmospheric gas also flows out from the upstream side exhaust purification catalyst 20. As a result, the output air-fuel ratio of the downstream air-fuel ratio sensor 41 also changes to a lean air-fuel ratio with a very high degree of lean corresponding to the atmospheric gas.

In the present embodiment, at time t _{1 when} execution of fuel cut control is started, the applied voltage V to the downstream air-fuel ratio sensor 41 is set to the second voltage to start the abnormality diagnosis of the downstream air-fuel ratio sensor 41. The voltage is raised to V ₂ (for example, 1.0 V). Here, the second voltage V ₂ is a voltage within the limit current region Wlc that is generated when atmospheric gas is circulating around the downstream air-fuel ratio sensor 41 when no abnormality occurs in the downstream air-fuel ratio sensor 41. It is.

Thereafter, in the example shown in FIG. 11, at time t ₂ , the increase in the output air-fuel ratio of the downstream side air-fuel ratio sensor 41 ends and converges to a constant value. In this embodiment, abnormality diagnosis is started at time t ₂ when the output air-fuel ratio of the downstream air-fuel ratio sensor 41 has converged, and the downstream air-fuel ratio sensor is set for a predetermined time Δt from this time t _2. The voltage applied to 41 is kept constant.

Thereafter, in the present embodiment, the applied voltage V to the downstream air-fuel ratio sensor 41 is the first voltage V ₁ (for example, 0.2 V) at time t ₃ when a predetermined time Δt has elapsed from time t _2. ). Here, the first voltage V ₁ is a voltage in the proportional region Wip that is generated when atmospheric gas is circulating around the downstream air-fuel ratio sensor 41 when no abnormality has occurred in the downstream air-fuel ratio sensor 41. is there. In the present embodiment, the voltage V applied to the downstream air-fuel ratio sensor 41 is applied to the downstream air-fuel ratio sensor 41 for a predetermined time Δt from time t ₃ when the voltage V is changed to the first voltage V _1. The voltage is kept constant.

In the example shown in FIG. 11, for abnormality diagnosis, at a time t ₄ when a predetermined time Δt has elapsed since the voltage V applied to the downstream air-fuel ratio sensor 41 is changed to the first voltage V _1. Detection of the output current I of the downstream air-fuel ratio sensor 41 is completed. Therefore, at time t ₄ , the voltage applied to the downstream air-fuel ratio sensor 41 is raised to the normal control voltage (for example, 0.45 V). In the example shown in FIG. 11, then the fuel cut control is made to finished at time t _5.

At time t _5, when the fuel cut control is made to completion, execution of the return after the rich control is started accordingly. For this reason, the target air-fuel ratio is set to the post-return rich set air-fuel ratio AFTrt that is richer than the rich set air-fuel ratio AFTrich. When the target air-fuel ratio becomes the rich set air-fuel ratio after return, the output air-fuel ratio of the upstream side air-fuel ratio sensor 40 also changes to the air-fuel ratio corresponding to the rich set air-fuel ratio AFTrt after return. The rich air-fuel ratio exhaust gas also flows into the upstream side exhaust purification catalyst 20, but unburned gas in the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is oxygen stored in the upstream side exhaust purification catalyst 20. It reacts with and is purified. As a result, the output air-fuel ratio of the downstream air-fuel ratio sensor 41 is reduced with execution of the return after the rich control at time t ₅ is started, eventually becomes substantially the stoichiometric air-fuel ratio.

Further, in the present embodiment, at time t ₅ that the fuel cut control is started, in order to start the abnormality diagnosis of the downstream air-fuel ratio sensor 41, the voltage V applied to the downstream-side air-fuel ratio sensor 41 is a fourth The voltage is V ₄ (for example, 0.45 V). Here, the fourth voltage V ₄ is a limit current region generated in a state where exhaust gas of the theoretical air-fuel ratio is circulating around the downstream air-fuel ratio sensor 41 when no abnormality occurs in the downstream air-fuel ratio sensor 41. Is the voltage inside.

Thereafter, in the example shown in FIG. 11, at time t ₆ , the decrease in the output air-fuel ratio of the downstream air-fuel ratio sensor 41 ends and converges to a constant value. In the present embodiment, the voltage applied to the downstream-side air-fuel ratio sensor 41 over a predetermined time Δt that the output air-fuel ratio of the downstream air-fuel ratio sensor 41 is determined in advance from the time t ₆ converged is maintained constant.

Then, in the present embodiment, at time t ₇ for a predetermined time Δt has passed a predetermined from the time t _6, the downstream air-fuel ratio voltage V applied to the sensor 41 is the third of the applied voltage V ₃ (e.g., 0. 1V). Here, the third voltage V ₃ is a proportional region generated in a state where exhaust gas having a theoretical air-fuel ratio is circulating around the downstream air-fuel ratio sensor 41 when no abnormality occurs in the downstream air-fuel ratio sensor 41. Is the voltage inside. In the present embodiment, the voltage V applied to the downstream air-fuel ratio sensor 41 is applied to the downstream air-fuel ratio sensor 41 for a predetermined time Δt from time t ₇ when the voltage V applied to the downstream air-fuel ratio sensor 41 is changed to the third voltage V _3. The voltage is kept constant.

In the example shown in FIG. 11, the abnormality diagnosis is completed at time t ₈ when a predetermined time Δt has elapsed from time t ₇ . Therefore, at time t ₈ , the voltage applied to the downstream air-fuel ratio sensor 41 is raised to the normal control voltage (for example, 0.45 V). Further, in the example shown in FIG. 11, since the return after the rich control is also at time t ₈ not completed, the target air-fuel ratio is maintained at the return after the rich set air-fuel ratio AFTrt. As a result, the output air-fuel ratio of the upstream side air-fuel ratio sensor 40 becomes a rich air-fuel ratio, and the oxygen storage amount of the upstream side exhaust purification catalyst 20 gradually decreases.

Thereafter, when the oxygen storage amount of the upstream side exhaust purification catalyst 20 gradually decreases, it eventually becomes almost zero, and the rich air-fuel ratio exhaust gas starts to flow out from the upstream side exhaust purification catalyst 20. Thus, the output air-fuel ratio of the downstream air-fuel ratio sensor 41 becomes equal to or lower than the rich determining the air-fuel ratio AFrich at time t _9. In the present embodiment, when the output air-fuel ratio of the downstream side air-fuel ratio sensor 41 becomes equal to or less than the rich determination air-fuel ratio AFrich as described above, the rich control after returning is terminated, and the normal control shown in FIG. 5 is resumed.

Here, in the present embodiment, when the downstream air-fuel ratio sensor 41 is normal, the downstream air-fuel ratio sensor 41 is in a state where the exhaust air-fuel ratio around the downstream air-fuel ratio sensor 41 is an air-fuel ratio corresponding to the atmospheric gas. The output current when the applied voltage I to 41 is the voltage V ₂ in the limit current region Wlc is detected or calculated in advance experimentally or by calculation as a normal value. Similarly, when the downstream air-fuel ratio sensor 41 is normal, the applied voltage to the downstream air-fuel ratio sensor 41 in a state where the exhaust air-fuel ratio around the downstream air-fuel ratio sensor 41 is an air-fuel ratio corresponding to atmospheric gas. The output current when I is the voltage V ₁ in the proportional region Wip is detected or calculated as a normal value experimentally or by calculation in advance.

When the control as shown in FIG. 11 is performed and the downstream air-fuel ratio sensor 41 is normal, the downstream air-fuel ratio sensor 41 is supplied with the voltage V ₂ in the limit current region as described above. The detected value of the output current I by the current detector 61 in the applied state substantially matches the normal value (normal value in the limit current region) in such a state. Similarly, when the downstream air-fuel ratio sensor 41 is normal, the output current I is detected by the current detector 61 with the voltage V ₁ in the proportional region applied to the downstream air-fuel ratio sensor 41 as described above. The value substantially matches the normal value (normal value outside the limit current region) in such a state. Therefore, in this embodiment, the detection value of the output current I of the downstream air-fuel ratio sensor 41 at time t ₂ ~t ₃ is almost identical with the corresponding limiting current region within normal values, and at time t ₃ ~t ₄ When the detected value of the output current I of the downstream side air-fuel ratio sensor 41 substantially matches the corresponding normal value outside the limit current region, it is determined that the downstream side air-fuel ratio sensor 41 is normal.

On the other hand, when an abnormality occurs in the circuit or the like of the downstream air-fuel ratio sensor 41, that is, when an offset shift occurs in the downstream air-fuel ratio sensor 41, as described above, the limit current is supplied to the downstream air-fuel ratio sensor 41. The detected value of the output current I by the current detection unit 61 in the state where the voltage V ₂ in the region is applied is a reference value in which a difference from the corresponding normal value in the limit current region is determined in advance (the reference value in the limit current region) ) It becomes a value that becomes above. Similarly, if the offset deviation occurs on the downstream-side air-fuel ratio sensor 41, the output current by the current detecting section 61 in the state of applying the voltages V ₁ in the proportional region downstream-side air-fuel ratio sensor 41 as described above The detected value of I is a value such that the difference from the corresponding normal value outside the limit current region is not less than a predetermined reference value (reference value outside the limit current region). Therefore, in the present embodiment, the difference between the detected value and the corresponding limiting current region within the normal value of the output current I of the downstream air-fuel ratio sensor 41 at time t ₂ ~t ₃ is equal to or greater than the reference value, and the time t ₃ If the difference between the detected value and the corresponding limiting current region outside the normal value of the output current I of the downstream air-fuel ratio sensor 41 in ~t ₄ is equal to or greater than the reference value, an offset shift in the downstream air-fuel ratio sensor 41 It is determined that it has occurred.

On the other hand, if there is an abnormality in the diffusion rate controlling layer 54 or the electrode 52 of the downstream air-fuel ratio sensor 41, that is, if there is a tilt shift in the downstream air-fuel ratio sensor 41, the downstream air-fuel ratio sensor 41 is The detected value of the output current I by the current detector 61 in a state where the voltage V ₂ in the limit current region is applied to the fuel ratio sensor 41 is a reference value (limit value) in which the difference from the corresponding normal value in the limit current region is predetermined. It becomes a value that is equal to or greater than the reference value in the current region. Similarly, if the displacement inclination to the downstream side air-fuel ratio sensor 41 has occurred, the output current by the current detection unit 61 in the state of applying the voltages V ₁ in the proportional region downstream-side air-fuel ratio sensor 41 as described above The detected value of I substantially matches the corresponding normal value outside the limit current region. Therefore, in the present embodiment, the difference between the detected value and the corresponding limiting current region within the normal value of the output current I of the downstream air-fuel ratio sensor 41 at time t ₂ ~t ₃ is equal to or greater than the reference value, and the time t ₃ When the detected value of the output current I of the downstream side air-fuel ratio sensor 41 at t ₄ substantially coincides with the corresponding normal value outside the limit current region, it is determined that the downstream air-fuel ratio sensor 41 has a tilt deviation. Is done.

Further, when an abnormality such as an element crack occurs in the downstream air-fuel ratio sensor 41, that is, when a reference gas abnormality occurs in the downstream air-fuel ratio sensor 41, as described above, The detected value of the output current I by the current detector 61 in a state where the voltage V ₂ in the limit current region is applied substantially matches the corresponding normal value in the limit current region. Similarly, if the displacement inclination to the downstream side air-fuel ratio sensor 41 has occurred, the output current by the current detection unit 61 in the state of applying the voltages V ₁ in the proportional region downstream-side air-fuel ratio sensor 41 as described above The detected value of I is a value such that the difference from the corresponding normal value outside the limit current region is not less than a predetermined reference value (reference value outside the limit current region). Therefore, in this embodiment, substantially coincides with the limiting current region within the normal value detection value corresponding the output current I of the downstream air-fuel ratio sensor 41 at time t ₂ ~t _3, and downstream at time t ₃ ~t ₄ When the difference between the detected value of the output current I of the side air-fuel ratio sensor 41 and the corresponding normal value outside the limit current region is greater than or equal to the reference value, the downstream air-fuel ratio sensor 41 has an abnormality in the reference gas. It is judged.

Similarly, on the basis of the output current I of the downstream air-fuel ratio sensor 41 detected in the output current I and the time t ₇ ~t ₈ of the downstream air-fuel ratio sensor 41 which is detected at time t ₆ ~t ₇ It is also possible to detect. Also in this case, when the downstream air-fuel ratio sensor 41 is normal, the applied voltage I to the downstream air-fuel ratio sensor 41 is in a state where the exhaust air-fuel ratio around the downstream air-fuel ratio sensor 41 is the stoichiometric air-fuel ratio. The output current at the voltage V ₄ in the limit current region is detected or calculated in advance experimentally or by calculation as a normal value in the limit current region. Similarly, when the downstream air-fuel ratio sensor 41 is normal and the exhaust air-fuel ratio around the downstream air-fuel ratio sensor 41 is the stoichiometric air-fuel ratio, the applied voltage I to the downstream air-fuel ratio sensor 41 is in the proportional range. The output current at the voltage V ₃ in Wip is detected or calculated in advance experimentally or by calculation as a normal value outside the limit current region.

When the control as shown in FIG. 11 is performed, the detected value of the output current I by the current detection unit 61 in a state where the voltage V ₄ in the limit current region is applied to the downstream air-fuel ratio sensor 41, and the correspondence The difference from the normal value in the limiting current region is calculated. In addition, the difference between the detected value of the output current I by the current detector 61 and the corresponding normal value outside the limit current region in a state where the voltage V ₃ in the proportional region is applied to the downstream air-fuel ratio sensor 41 is calculated. . Based on the difference between the output currents I calculated in this way, the abnormal mode of the downstream side air-fuel ratio sensor 41 is diagnosed by a method similar to that at the times t _{2 to} t ₄ described above.

In the above embodiment, abnormality diagnosis is performed twice at times t _{2 to} t ₄ during execution of fuel cut control and at times t _{6 to} t ₈ during execution of rich control after return. However, the abnormality diagnosis of the downstream air-fuel ratio sensor 41 may be only one of them.

  In the above embodiment, the abnormality diagnosis of the downstream air-fuel ratio sensor 41 has been described as an example. However, the abnormality diagnosis of the upstream air-fuel ratio sensor 40 can be performed in the same manner. However, the exhaust gas before flowing into the upstream side exhaust purification catalyst 20 flows around the upstream side air-fuel ratio sensor 40 during execution of the rich control after the return. Therefore, it is unclear what the air-fuel ratio of the exhaust gas flowing around the upstream air-fuel ratio sensor 40 is during the rich control after return. For this reason, the abnormality diagnosis of the upstream air-fuel ratio sensor 40 is not performed during the execution of rich control after return.

  Furthermore, in the above embodiment, one voltage in the limit current region and one voltage in the proportional region are applied to the downstream air-fuel ratio sensor 41, and based on the output current I of the air-fuel ratio sensors 40 and 41 at this time. Thus, the type of abnormality of the air-fuel ratio sensors 40 and 41 is determined. However, a plurality of different voltages may be applied in the limit current region and the proportional region, respectively, or a plurality of different voltages may be applied only in one of the limit current region and the proportional region. May be. Here, the output current I does not change even if the applied voltage V changes basically within the limit current region, but the output current I also changes when the applied voltage V changes within the proportional region. For this reason, it is preferable that the number of different voltages applied in the proportional region is larger than the number of different voltages applied in the limit current region.

  According to this embodiment, as described above, different abnormalities are detected by detecting the output current of the air-fuel ratio sensor in a state where the voltage in the limit current region and the voltage in the proportional region are applied to the air-fuel ratio sensors 40, 41. The mode can be distinguished particularly from an abnormality due to an offset deviation and an abnormality due to other causes.

<Flowchart>
FIG. 12 shows a flowchart of a control routine for diagnosing abnormality of the downstream air-fuel ratio sensor 41. In particular, FIG. 12 shows a flowchart when an abnormality diagnosis is performed during execution of the fuel cut control, that is, when an abnormality diagnosis is performed at times t _{2 to} t ₄ in FIG. The illustrated control routine is performed by interruption at regular time intervals.

  First, in step S11, it is determined whether or not an abnormality diagnosis execution condition is satisfied. When the condition for executing the abnormality diagnosis is satisfied, for example, the temperature of the downstream air-fuel ratio sensor 41 is equal to or higher than its activation temperature and the ignition key of the vehicle equipped with the internal combustion engine is turned on after the internal combustion engine is started. This is established when the abnormality diagnosis of the downstream air-fuel ratio sensor 41 has not been completed. If it is determined in step S11 that the abnormality diagnosis execution condition is not satisfied, the process proceeds to step S12. In step S12, the number of application times i of different voltages to be described later is reset to 1, the output currents I (1) to I (n) at the time of application of the first to nth voltages are reset to 0, and the control routine is terminated. It is done.

  On the other hand, if it is determined in step S11 that the abnormality diagnosis execution condition is satisfied, the process proceeds to step S13. In step S13, it is determined whether fuel cut control (FC) is being executed. If it is determined in step S13 that the fuel cut control is not being executed, the process proceeds to step S12, where the voltage application count i is reset to 1, and the output current at the time of the first to n-th voltage application is 0. To reset the control routine.

  Thereafter, when execution of fuel cut control is started, the process proceeds from step S13 to step S14 in the next control routine. In step S14, the applied voltage V to the downstream air-fuel ratio sensor 41 is set to the i-th applied voltage V (i). Here, the i-th applied voltage V (i) is set in advance. For example, the first applied voltage V (1) is a limit that occurs when air gas is circulating around the air-fuel ratio sensors 40, 41 when no abnormality occurs in the air-fuel ratio sensors 40, 41. The voltage is in the current region. In addition, the second applied voltage V (2) is generated in a state where air gas is circulating around the air-fuel ratio sensors 40, 41 when no abnormality has occurred in the air-fuel ratio sensors 40, 41. The voltage is within the proportional range. Note that the number of times i and the i-th applied voltage V (i) of different voltages are applied as long as the voltage in the limit current region is applied at least once and the voltage in the proportional region is applied at least once. The voltage may be set.

Here, before the start of the fuel cut control, the voltage application count i is set to 1 in step S12. Therefore, immediately after the start of the fuel cut control, the voltage application count i is set to 1 in step S14. For this reason, immediately after the start of the fuel cut control, the applied voltage V is the first applied voltage V (1), for example, the voltage V ₂ in the limit current region. Next, in step S15, it is determined whether or not the output current I of the downstream air-fuel ratio sensor 41 has become stable. Whether or not the output current I of the downstream air-fuel ratio sensor 41 has stabilized is determined based on, for example, whether or not the amount of change in the output current I of the downstream air-fuel ratio sensor 41 per unit time has become a certain amount or less. Is done. Alternatively, whether or not the output current I of the downstream side air-fuel ratio sensor 41 is stable may be determined based on whether or not the elapsed time after changing the applied voltage V is equal to or longer than a predetermined time. Good.

  If it is determined in step S15 that the output current I of the downstream air-fuel ratio sensor 41 is not stable, the control routine is terminated. On the other hand, when the output current I of the downstream air-fuel ratio sensor 41 is stabilized, the process proceeds from step S15 to step S16. In step S16, it is determined whether or not the elapsed time since the output current I of the downstream air-fuel ratio sensor 41 has been stabilized in step S15 is equal to or longer than a predetermined time Δt. If it is determined in step S16 that the elapsed time is shorter than the predetermined time Δt, the control routine is terminated.

  On the other hand, when a predetermined time Δt has elapsed after it has been determined that the output current I of the downstream air-fuel ratio sensor 41 has stabilized, the process proceeds from step S16 to step S17 in the next control routine. In step S17, an average value of the output current I of the downstream side air-fuel ratio sensor 41 from when it is determined that the output current I of the downstream side air-fuel ratio sensor 41 is stable until a predetermined time Δt elapses is calculated. Is the output current I (i) when the i-th applied voltage V (i) is applied. Therefore, when the first applied voltage V (1) is applied, the output current I (1) when the first applied voltage V (1) is applied is calculated.

  Next, in step S18, it is determined whether the number of application times i of different voltages is n or more. n is a value of 2 or more. If the current application number i of different voltages is less than n, the process proceeds to step S19. In step S19, 1 is added to the number of application times i of different voltages, and the control routine is terminated.

  When 1 is added to the number of application times i of different voltages and the number of application times of different voltages becomes 2, in the next control routine, the applied voltage V is set to the second applied voltage V (2) in step S14. After that, after the applied voltage V is set to the second applied voltage V (2) and the elapsed time after it is determined that the output current I of the downstream air-fuel ratio sensor 41 is stabilized becomes equal to or longer than the predetermined time Δt, Proceed to step S17. In step S17, an average value of the output current I of the downstream side air-fuel ratio sensor 41 from the time when it is determined that the output current I of the downstream side air-fuel ratio sensor 41 has stabilized until a predetermined time Δt elapses is calculated. The output current I (2) when the second applied voltage V (2) is applied.

  Next, in step S18, it is determined whether or not the number of application times i of different voltages is n or more. When n is 2, it is determined that the number of application times i of different voltages is n or more. . On the other hand, when n is 3 or more, steps S11 to S17 are repeated until the number of different voltage applications is n. If it is determined in step S18 that the number of application times i of different voltages is n or more, the process proceeds to step S20.

  In step S20, based on the output currents I (0) to I (n) calculated in step S17, these are compared with normal values as described above, and the abnormal mode of the downstream air-fuel ratio sensor 41 is determined. The Next, in step S21, the number i of application of different voltages is reset to 1, the output current at the time of the first to n-th voltage application is reset to 0, and the control routine is terminated.

  The control routine shown in FIG. 12 shows a case where abnormality diagnosis is performed during execution of fuel cut control. However, when abnormality diagnosis is performed during execution of rich control after return, the same control routine performs abnormality diagnosis. Diagnosis can be made. In this case, in step S13, it is determined whether or not the post-return rich control is being executed, not whether or not the fuel cut control is being executed. In this case, the i-th applied voltage V (i) is also different from the applied voltage when the fuel cut control is being executed.

<Second embodiment>
Next, with reference to FIG.13 and FIG.14, the abnormality diagnosis apparatus which concerns on 2nd embodiment of this invention is demonstrated. The configuration and control in the abnormality diagnosis device according to the second embodiment are basically the same as the configuration and control in the abnormality diagnosis device according to the first embodiment, except for the parts described below.

  By the way, when the upstream air-fuel ratio sensor 40 is not abnormal, the feedback control is performed so that the output air-fuel ratio of the upstream air-fuel ratio sensor 40 becomes the target air-fuel ratio. The air-fuel ratio of the exhaust gas flowing into the air-fuel ratio is the same as the target air-fuel ratio. Therefore, when the target air-fuel ratio is kept constant at the stoichiometric air-fuel ratio, the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 becomes the stoichiometric air-fuel ratio, and the exhaust gas flowing around the downstream air-fuel ratio sensor 41 The air-fuel ratio is also kept constant at the stoichiometric air-fuel ratio.

  Further, when the target air-fuel ratio is kept constant at the rich air-fuel ratio, unburned gas in the exhaust gas flowing in the upstream side exhaust purification catalyst 20 is purified in the upstream side exhaust purification catalyst 20. For this reason, when the target air-fuel ratio starts to be maintained at the rich air-fuel ratio, the air-fuel ratio of the exhaust gas flowing around the downstream air-fuel ratio sensor 41 becomes substantially the stoichiometric air-fuel ratio. However, when the oxygen storage amount of the upstream side exhaust purification catalyst 20 becomes zero, the upstream side exhaust purification catalyst 20 no longer purifies unburned gas. Therefore, finally, the air-fuel ratio of the exhaust gas flowing around the downstream air-fuel ratio sensor 41 is kept constant at the target air-fuel ratio that is a rich air-fuel ratio.

  When the abnormality diagnosis of the downstream air-fuel ratio sensor 41 is performed, as long as feedback control is performed so that the output air-fuel ratio of the upstream air-fuel ratio sensor 40 becomes the target air-fuel ratio, the area around the downstream air-fuel ratio sensor 41 is circulated. The air-fuel ratio of the exhaust gas can be kept constant at the target air-fuel ratio. Therefore, in the present embodiment, the air-fuel ratio of the exhaust gas flowing around the downstream air-fuel ratio sensor 41 is maintained at a predetermined constant air-fuel ratio by maintaining the target air-fuel ratio constant at a predetermined air-fuel ratio. The downstream side air-fuel ratio sensor 41 is diagnosed for abnormality.

  Next, referring to the time chart shown in FIG. 13, the abnormality diagnosis of the downstream air-fuel ratio sensor 41 in the present embodiment will be described by taking as an example the case where the target air-fuel ratio is maintained at the stoichiometric air-fuel ratio. FIG. 13 is a time chart of the abnormality diagnosis flag, the target air-fuel ratio, the output air-fuel ratio of the upstream air-fuel ratio sensor 40, the output air-fuel ratio of the downstream air-fuel ratio sensor 41, and the voltage applied to the downstream air-fuel ratio sensor 41.

Also in this embodiment, as already described with reference to FIG. 5, the target air-fuel ratio is normally changed alternately to the rich set air-fuel ratio AFTrich and the lean set air-fuel ratio AFTlean. In the example shown in FIG. 13, before the target air-fuel ratio is made the stoichiometric air-fuel ratio at the time t ₁ to start the abnormality diagnosis, the target air-fuel ratio is changed to the rich air-fuel ratio and the lean air-fuel ratio alternately. This shows a case where the target air-fuel ratio is the rich set air-fuel ratio AFTrich during control.

In the example shown in FIG. 13, at time t ₁ , the target air-fuel ratio is changed from the rich set air-fuel ratio AFTrich to the stoichiometric air-fuel ratio (14.6) in order to start abnormality diagnosis. Along with this, the output air-fuel ratio AFup of the upstream air-fuel ratio sensor changes to the stoichiometric air-fuel ratio. On the other hand, the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is maintained as the stoichiometric air-fuel ratio. In the present embodiment, when abnormality diagnosis is started, the voltage V applied to the downstream air-fuel ratio sensor 41 is set to the fourth current V ₄ (for example, 0.45 V). Here, the fourth voltage V ₄ is a limit current region generated in a state where exhaust gas of the theoretical air-fuel ratio is circulating around the downstream air-fuel ratio sensor 41 when no abnormality occurs in the downstream air-fuel ratio sensor 41. Is the voltage inside.

Thereafter, in the present embodiment, the applied voltage to the downstream air-fuel ratio sensor 41 is kept constant for a predetermined time Δt from time t ₂ when a predetermined time Δt _{0 has} elapsed from time t _1. . Here, at the time Δt ₀ , for example, even when the output air-fuel ratio of the downstream air-fuel ratio sensor 41 is a rich air-fuel ratio at the time t ₁ , the target air-fuel ratio is changed to the stoichiometric air-fuel ratio. As a result, the time required for the output air-fuel ratio of the downstream air-fuel ratio sensor 41 to converge to the stoichiometric air-fuel ratio is set.

Then, in the present embodiment, at time t ₃ when the predetermined time Δt has passed a predetermined from time t _2, the voltage V applied to the downstream-side air-fuel ratio sensor 41 is the third voltage V ₃ (e.g., 0.1 V ). Here, the third voltage V ₃ is a proportional region Wip generated in a state where exhaust gas of the stoichiometric air-fuel ratio is circulating around the downstream air-fuel ratio sensor 41 when no abnormality has occurred in the downstream air-fuel ratio sensor 41. Is the voltage inside. In the present embodiment, the application to the downstream air-fuel ratio sensor 41 is performed for a predetermined time Δt from time t ₃ when the applied voltage V to the downstream air-fuel ratio sensor 41 is changed to the third voltage V _3. The voltage is kept constant.

In the example shown in FIG. 13, at time t ₄ when a certain time Δt predetermined from the time t ₃ has elapsed, the abnormality diagnosis is completed. Therefore, at time t ₄ , the voltage applied to the downstream side air-fuel ratio sensor 41 is raised to a voltage for normal control (for example, 0.45 V), and the target air-fuel ratio is also returned to the rich set air-fuel ratio AFTrich. The normal control shown in FIG. 5 is performed.

Here, also in the present embodiment, when the downstream air-fuel ratio sensor 41 is normal, the exhaust air-fuel ratio around the downstream air-fuel ratio sensor 41 is in the state where the stoichiometric air-fuel ratio is the stoichiometric air-fuel ratio. The output current when the applied voltage I is the voltage V ₄ in the limit current region is detected or calculated in advance experimentally or by calculation as a normal value in the limit current region. Similarly, when the downstream air-fuel ratio sensor 41 is normal and the exhaust air-fuel ratio around the downstream air-fuel ratio sensor 41 is the stoichiometric air-fuel ratio, the applied voltage I to the downstream air-fuel ratio sensor 41 is in the proportional range. The output current at the voltage V ₃ is detected or calculated in advance experimentally or by calculation as a normal value outside the limit current region.

When the control as shown in FIG. 13 is performed, the detected value of the output current I of the downstream air-fuel ratio sensor 41 at the times t _{2 to} t ₃ substantially coincides with the corresponding normal value in the limit current region, If the detected value of the output current I of the downstream air-fuel ratio sensor 41 at times t _{3 to} t ₄ substantially matches the corresponding normal value outside the limit current region, it is determined that the downstream air-fuel ratio sensor 41 is normal. Is done. Further, the difference between the detected value of the output current I of the downstream side air-fuel ratio sensor 41 at the time t _{2 to} t ₃ and the corresponding normal value in the limit current region is equal to or greater than the reference value, and the downstream at the time t _{3 to} t ₄ . If the difference between the detected value of the output current I of the side air-fuel ratio sensor 41 and the corresponding normal value outside the limit current region is equal to or greater than the reference value, it is determined that the downstream air-fuel ratio sensor 41 has an offset deviation. Is done.

On the other hand, the difference between the detected value of the output current I of the downstream air-fuel ratio sensor 41 at times t _{2 to} t ₃ and the corresponding normal value in the limit current region is equal to or greater than the reference value, and the downstream at times t _{3 to} t ₄ . If the detected value of the output current I of the side air-fuel ratio sensor 41 substantially matches the corresponding normal value outside the limit current region, it is determined that the downstream air-fuel ratio sensor 41 has a tilt shift. Further, the time t ₂ substantially coincides with the limiting current region within the normal value detection value corresponding the output current I of the downstream air-fuel ratio sensor 41 in ~t _3, the downstream air-fuel ratio sensor 41 at and time t ₃ ~t ₄ When the difference between the detected value of the output current I and the corresponding normal value outside the limit current region is equal to or larger than the reference value, it is determined that the downstream air-fuel ratio sensor 41 has a reference gas abnormality.

FIG. 13 shows a case where the target air-fuel ratio is maintained constant at the stoichiometric air-fuel ratio, but the target air-fuel ratio may be maintained at an air-fuel ratio other than the stoichiometric air-fuel ratio. However, in this case, the oxygen storage amount of the upstream side exhaust purification catalyst 20 needs to reach the maximum storable oxygen amount or zero until the air-fuel ratio of the exhaust gas flowing around the downstream side air-fuel ratio sensor 41 becomes stable. . For this reason, the time Δt _0, which is the time required for the air-fuel ratio of the exhaust gas flowing around the downstream air-fuel ratio sensor 41 to converge, is a relatively long time.

  According to this embodiment, as described above, different abnormalities are detected by detecting the output current of the air-fuel ratio sensor in a state where the voltage in the limit current region and the voltage in the proportional region are applied to the air-fuel ratio sensors 40, 41. The mode can be distinguished particularly from an abnormality due to an offset deviation and an abnormality due to other causes.

  In the first embodiment, an abnormality diagnosis is performed during fuel cut control or during return rich control. However, the fuel cut control and the post-return rich control are executed according to the engine operating state, and may not be executed for a long period of time depending on circumstances. For this reason, abnormality diagnosis may not be executed over a long period of time. On the other hand, in the present embodiment, the normal control may be temporarily interrupted to maintain the target air-fuel ratio at a constant value, so that abnormality diagnosis can be performed at any timing.

  In the second embodiment, when performing abnormality diagnosis, the target air-fuel ratio is maintained at a predetermined constant air-fuel ratio. However, when performing abnormality diagnosis, the target air-fuel ratio may be switched between the rich air-fuel ratio and the lean air-fuel ratio alternately at short intervals. When the target air-fuel ratio is switched between the rich air-fuel ratio and the lean air-fuel ratio alternately at short intervals, unburned gas and air in the exhaust gas are removed by the upstream side exhaust purification catalyst 20. For this reason, the air-fuel ratio of the exhaust gas flowing around the downstream air-fuel ratio sensor 41 is maintained constant at the stoichiometric air-fuel ratio. In this case, the target air-fuel ratio is alternated between the rich air-fuel ratio and the lean air-fuel ratio so that the oxygen storage amount of the upstream side exhaust purification catalyst 20 is maintained to be greater than zero and less than the maximum storable oxygen amount. It is necessary to change to.

<Flowchart>
FIG. 14 shows a flowchart of a control routine for performing abnormality diagnosis of the downstream air-fuel ratio sensor 41. The illustrated control routine is performed by interruption at regular time intervals.

  As shown in FIG. 14, first, in step S31, it is determined whether or not an abnormality diagnosis execution condition is satisfied. If it is determined in step S31 that the abnormality diagnosis execution condition is not satisfied, the process proceeds to step S32. In step S32, the number of application times i of different voltages is reset to 1, output currents I (0) to I (n) at the time of application of the first to nth voltages are reset to 0, and the control routine is terminated.

On the other hand, if it is determined in step S32 that the abnormality diagnosis execution condition is not satisfied, the process proceeds to step S33. In step S33, the target air-fuel ratio is set to the theoretical air-fuel ratio (14.6). Next, in step S34, as in step S14, the applied voltage V to the downstream air-fuel ratio sensor 41 is set to the i-th applied voltage V (i). Next, in step S35, it is determined whether or not the number of application times i of different voltages is 2 or more. If the number of application times i is 1, the process proceeds to step S36. In step S36, it is determined whether or not an elapsed time after setting the target air-fuel ratio to the stoichiometric air-fuel ratio is equal to or greater than the predetermined time Δt ₀ described above. In step S36, when it is determined that the elapsed time since the target air-fuel ratio is set to the stoichiometric air-fuel ratio is less than the predetermined time Δt ₀ described above, that is, the exhaust gas flowing around the downstream air-fuel ratio sensor 41 is exhausted. If it is determined that the air-fuel ratio may not be stable, the control routine is terminated.

On the other hand, if it is determined in step S36 that the elapsed time is equal to or longer than the predetermined time Δt ₀ , the process proceeds from step S36 to step S37. In step S37, whether or not the elapsed time since the target air-fuel ratio is set to the stoichiometric air-fuel ratio is not less than a predetermined time Δt ₀ is not less than a predetermined time Δt. Is determined. If it is determined in step S37 that the elapsed time is equal to or longer than the predetermined time Δt, the process proceeds from step S37 to step S38. In step S38, the average value of the output current I of the downstream side air-fuel ratio sensor 41 is calculated during the lapse of the fixed time Δt, and this average value is the output current when the i-th applied voltage V (i) is applied. I (i). Next, in step S39, it is determined whether or not the number of application times i of different voltages is n or more. If the current application number i of the different voltage is smaller than n, the process proceeds to step S40. In step S40, 1 is added to the number of application times i of different voltages, and the control routine is terminated.

  When 1 is added to the number of application times i of different voltages and the number of application times of different voltages becomes 2, the process proceeds from step S35 to step S41 in the next control routine. In step S41, it is determined whether or not the output current I of the downstream air-fuel ratio sensor 41 has stabilized after the applied voltage is changed. If it is determined in step S35 that the output current I of the downstream air-fuel ratio sensor 41 is not stable, the control routine is ended. On the other hand, when the output current I of the downstream air-fuel ratio sensor 41 is stabilized, the process proceeds from step S41 to step S37. Thereafter, the process proceeds to step S39 via steps S37 and S38. In step S39, it is determined again whether or not the number of application times i of different voltages is n or more. When n is 2, it is determined that the number of application times i of different voltages is n or more. On the other hand, when n is 3 or more, steps S31 to S38 are repeated until the number of application times of different voltages is n. If it is determined in step S39 that the number of application times i of different voltages is n or more, the process proceeds to step S42.

  In step S42, based on the output currents I (0) to I (n) calculated in step S38, these are compared with normal values as described above, and the abnormal mode of the downstream air-fuel ratio sensor 41 is determined. The Next, in step S43, the number i of application of different voltages is reset to 1, and the output current at the time of application of the first to nth voltages is reset to 0. Next, in step S44, the target air-fuel ratio is set to the target air-fuel ratio in normal control, and the control routine is ended.

DESCRIPTION OF SYMBOLS 1 Engine body 5 Combustion chamber 7 Intake port 9 Exhaust port 19 Exhaust manifold 20 Upstream exhaust purification catalyst 24 Downstream exhaust purification catalyst 31 ECU
40 upstream air-fuel ratio sensor 41 downstream air-fuel ratio sensor

Claims (11)

In an abnormality diagnosis apparatus for an air-fuel ratio sensor that is provided in an exhaust passage of an internal combustion engine and generates a limit current corresponding to the air-fuel ratio,
A current detector that detects an output current of the air-fuel ratio sensor; and an applied voltage control device that controls an applied voltage to the air-fuel ratio sensor.
A voltage within a limit current region where a limit current is generated in the air-fuel ratio sensor and a voltage outside the limit current region when the air-fuel ratio of the exhaust gas flowing around the air-fuel ratio sensor is set to a predetermined constant air-fuel ratio; An abnormality diagnosis device for an air-fuel ratio sensor that determines the type of abnormality occurring in the air-fuel ratio sensor based on the output current of the air-fuel ratio sensor detected by a current detection unit at this time.   2. The abnormality diagnosis device for an air-fuel ratio sensor according to claim 1, wherein the voltage outside the limit current region is a voltage within a proportional region that is lower than the limit current region and in which the output current increases as the applied voltage increases. . When the air-fuel ratio sensor is normal, the air-fuel ratio sensor is maintained in the limit current region while the air-fuel ratio of the exhaust gas flowing around the air-fuel ratio sensor is maintained at the predetermined constant air-fuel ratio. Output current when applying a voltage of and the voltage outside the limit current region is detected or calculated in advance as a normal value within the limit current region and a normal value outside the limit current region, respectively,
With the air-fuel ratio of the exhaust gas flowing around the air-fuel ratio sensor maintained at the predetermined constant air-fuel ratio, the air-fuel ratio sensor is supplied with a voltage within the limit current region and a voltage outside the limit current region. The type of abnormality occurring in the air-fuel ratio sensor is determined based on the difference between the detected value of the output current of the air-fuel ratio sensor when the voltage is applied and the normal value in the limit current region and the normal value outside the limit current region The abnormality diagnosis device for an air-fuel ratio sensor according to claim 1 or 2.   The air-fuel ratio sensor when a voltage within the limit current region is applied to the air-fuel ratio sensor in a state where the air-fuel ratio of the exhaust gas flowing around the air-fuel ratio sensor is maintained at the predetermined constant air-fuel ratio The difference between the detected value of the output current and the normal value in the limit current region is equal to or greater than a predetermined reference value in the limit current region, and the air-fuel ratio of the exhaust gas flowing around the air-fuel ratio sensor is determined in advance. The difference between the detected value of the output current of the air-fuel ratio sensor and the normal value outside the limit current area when a voltage outside the limit current area is applied to the air-fuel ratio sensor while being maintained at a constant air-fuel ratio. Is equal to or greater than a predetermined reference value outside the limit current region, an offset in which the output current of the air-fuel ratio sensor is totally deviated from the air-fuel ratio of the exhaust gas flowing around the air-fuel ratio sensor Before the gap It is determined to be occurring in the air-fuel ratio sensor, abnormality diagnosis device for an air-fuel ratio sensor according to claim 3.   The air-fuel ratio sensor when a voltage within the limit current region is applied to the air-fuel ratio sensor in a state where the air-fuel ratio of the exhaust gas flowing around the air-fuel ratio sensor is maintained at the predetermined constant air-fuel ratio The difference between the detected value of the output current and the normal value in the limit current region is equal to or greater than a predetermined reference value in the limit current region, and the air-fuel ratio of the exhaust gas flowing around the air-fuel ratio sensor is The detected value of the output current of the air-fuel ratio sensor and the normal value outside the limit current area when a voltage outside the limit current area is applied to the air-fuel ratio sensor while being maintained at a predetermined constant air-fuel ratio. When the difference is less than a predetermined reference value outside the limit current region, the degree of change in the output current of the air-fuel ratio sensor with respect to the change in the air-fuel ratio of the exhaust gas flowing around the air-fuel ratio sensor is shifted. Slope Les is determined to have occurred in the air-fuel ratio sensor, abnormality diagnosis device for an air-fuel ratio sensor according to claim 3 or 4.   The internal combustion engine includes an exhaust purification catalyst disposed in the exhaust passage, an upstream air-fuel ratio sensor disposed in the exhaust passage upstream of the exhaust purification catalyst in the exhaust flow direction, and an exhaust flow direction of the exhaust purification catalyst. 6. A downstream air-fuel ratio sensor disposed in the exhaust passage on the downstream side, wherein the downstream air-fuel ratio sensor comprises the limit current type air-fuel ratio sensor. An air-fuel ratio sensor abnormality diagnosis device.   The internal combustion engine includes an exhaust purification catalyst disposed in the exhaust passage, an upstream air-fuel ratio sensor disposed in the exhaust passage upstream of the exhaust purification catalyst in the exhaust flow direction, and an exhaust flow direction of the exhaust purification catalyst. 6. A downstream air-fuel ratio sensor disposed in the exhaust passage on the downstream side, wherein the upstream air-fuel ratio sensor is composed of the limit current type air-fuel ratio sensor. An air-fuel ratio sensor abnormality diagnosis device. The internal combustion engine is capable of executing fuel cut control for stopping the supply of fuel to the combustion chamber during operation of the internal combustion engine,
8. The fuel cut control is being executed when the air-fuel ratio of the exhaust gas flowing around the air-fuel ratio sensor is maintained at the predetermined constant air-fuel ratio. The abnormality diagnosis device for an air-fuel ratio sensor according to the item. The internal combustion engine has a fuel cut control for stopping the supply of fuel to the combustion chamber during operation of the internal combustion engine, and an air / fuel ratio of the exhaust gas flowing into the exhaust purification catalyst after the completion of the fuel cut control. It is possible to execute post-return rich control that controls to a richer rich air-fuel ratio,
The air-fuel ratio according to claim 7, wherein the post-return rich control is being executed when the air-fuel ratio of the exhaust gas flowing around the air-fuel ratio sensor is maintained at the predetermined constant air-fuel ratio. Sensor abnormality diagnosis device. The internal combustion engine performs feedback control so that the output air-fuel ratio of the upstream air-fuel ratio sensor becomes a target air-fuel ratio,
The air-fuel ratio of the exhaust gas flowing around the air-fuel ratio sensor is maintained at the predetermined constant air-fuel ratio when the target air-fuel ratio is maintained constant at the predetermined air-fuel ratio. The abnormality diagnosis device for an air-fuel ratio sensor according to claim 7. The internal combustion engine performs feedback control so that the output air-fuel ratio of the upstream air-fuel ratio sensor becomes a target air-fuel ratio,
When the air-fuel ratio of the exhaust gas flowing around the air-fuel ratio sensor is maintained at the predetermined constant air-fuel ratio, the oxygen storage amount of the exhaust purification catalyst is greater than zero and the maximum storable oxygen amount When the target air-fuel ratio is alternately changed between a rich air-fuel ratio richer than the stoichiometric air-fuel ratio and a lean air-fuel ratio leaner than the stoichiometric air-fuel ratio so as to be maintained at a smaller amount. The abnormality diagnosis device for an air-fuel ratio sensor according to claim 7.

Images